In at least one known medical imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient, and after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile. Such a medical imaging system typically is referred to as a computed tomography (CT) system.
In known third generation CT systems, the x-ray source and the detector array are located on a rotatable gantry. The gantry rotates around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle are referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, projection data are processed to construct an image that corresponds to a two dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units". which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
The x-ray source, sometimes referred to as an x-ray tube, typically includes an evacuated glass x-ray envelope containing an anode and a cathode. X-rays are produced by applying a high voltage across the anode and cathode and accelerating electrons from the cathode against a focal spot on the anode. The x-rays produced by the x-ray tube diverge from the focal spot in a generally conical pattern.
Known cathode assemblies typically include a cathode cup and several current carrying filaments. Each filament includes a coil and leads extending from respective ends of the coil. The cathode cup has a filament receiving channel for each filament, and lead openings extend through the cup to each filament receiving channel.
To form the cathode assembly, filaments are inserted into the cathode cup so that each filament coil rests in a channel and the filament leads extend through the lead openings. Precise location of the filaments within the filament channels is important because such location affects operation characteristics of the x-ray tube, such as focal spot focusing. Incorrect focal spot focusing causes resolution loss and image degradation. Furthermore, if the filament is not properly positioned within a channel, the filament life may be shortened due, for example, to overheating. Accordingly, it is desirable to properly position each filament within its respective channel.
To facilitate proper positioning of each filament, filament inserts are positioned in each filament receiving channel of the cup. The inserts typically are positioned adjacent respective filament lead openings so that when a filament is inserted into the channel, the filament coil extends between the filament inserts and the filament leads extend through the filament lead openings. The inserts facilitate positioning of the filament and ensuring that each filament coil is centered within each channel.
Typically, each filament insert is welded into a respective channel. The inserts must also be precisely positioned within the channel to ensure proper filament positioning as described above. Such precise positioning of the inserts, and welding of the inserts, is time consuming and cumbersome.
It would be desirable to eliminate the time consuming and cumbersome process of positioning and welding inserts within the filament receiving channels of a cathode cup. Eliminating such inserts, however, preferably would not result in any less precise positioning of filaments within the filament receiving channels.